The performance, in particular the energy density, of electrochemical energy storage systems such as lithium-ion batteries (LIB) is generally a function of the choice and realization of the electrodes in the cell. Conventionally, two fundamentally different methods are described for coating the current collector with the electrode active material (also called active material hereinafter), namely the application of an active material slurry (the so-called slurry method), and the application of a free-standing active material foil.
Production methods using the slurry method are described for example in Japan Patent Application No. JP 2011-014262 and U.S. Patent Appl. Pub. No. 2014/0004418. In the production of active material slurries, in particular when the production process is at a standstill difficulties result because the mixing facilities have to be able to process large quantities of active material slurries, and separation of the components occurs quickly when mixing is inadequate.
The production of electrodes from free-standing active material foils is described for example in European Patent No. EP 1 644 136, U.S. Patent Appl. Pub. No. 2015/0061176 A1, U.S. Patent Appl. Pub. No. 2015/0062779 A1, U.S. Pat. No. 7,087,348 B2, U.S. Patent Appl. Pub. No. 2015/0072234, and U.S. Patent Appl. Pub. No. 2014/0127570. The free-standing active material foil is produced in a solvent-free method, standardly with a layer thickness of approximately 100-300 μm. The free-standing foil is cut to the desired size as needed, and is subsequently applied onto a pre-shaped current collector. The cutting of the electrode can also take place in a laminated composite of current collector and active material foil.
Conventionally, in the production of the free-standing active material foil an active material composition is provided including at least one electrode active material and at least one particulate binding agent, as well as, if warranted, at least one conductive additive, through the introduction of shear forces (e.g., through the use of mechanical mills such as jet or ball mills), forming fibrils from the binding agent particles. The compound can be shaped to form a stable, free-standing active material foil, e.g., with the aid of an extruder and/or a calendar.
European Patent No. EP 1 644 136 uses a jet mill for the fibrillation of the binding agent. Here, due to the collisions with other particles, the polymer binding agent experiences high shear forces such that a plasticization of the polymer occurs at least locally. This polymer then adheres to the particle surfaces with which the collision took place, in particular on the active material particles. In this method, it is problematic that in the jet mill agglomerates of the polymer binding agent first have to be broken up. For this purpose, low temperatures are necessary, in particular temperatures below the glass transition temperature Tg of the binding agent. On the other hand, a plasticization of the binding agent that is as good as possible is desired. However, this requires temperatures above the Tg of the polymer. In the existing art, the binding agent is therefore first mixed with the conductive additive in a mill having a rotary mixing tool in order to break up the polymer agglomerates through the filling of the surface of the binding agent particles. The subsequent fibrillation process in the jet mill then takes place at temperatures above the Tg of the binding agent. However, this procedure requires an additional method step.
U.S. Patent Appl. Pub. No. 2015/0062779 A1 describes first the cooling of an active material composition including active material particles, conductive additive, and binding agent particles, in order in this way to bring about a breaking up of the agglomerate of binding agent particles ahead of time in a mechanical mill having a rotary mixing tool. This is followed by the targeted fibrillation of the binding agent in a jet mill.
In order to make it possible to carry out a method that is as continuous as possible, it is desirable to keep the runup and rundown of the mixing duration until the desired product is obtained as short as possible, and to achieve a stationary state of the fibrillation step as quickly as possible. This is important in particular because the grinding process carried out in the jet mill leads over the long term to disintegration of the contained particles. Recycling of the reject product produced in the runup and rundown of the mixing process is therefore possible only to a very limited extent, because the portion of fine particles in the active material composition increases during a longer milling process, which has a disadvantageous effect on the performance of the product. The production of consistent products can be realized only with difficulty. Moreover, in jet mills there are different dwell times of the components of the composition. Because the grinding performance of large jet mills is generally better than the performance of small jet mills, methods using jet mills are difficult to apply on a large scale. Here, the increased grinding power has a negative effect on the product characteristics.
U.S. Patent Appl. Pub. No. 2015/0061176 A1 uses a classical form of mixing technology, the components of the active material composition being supplied to a ball mill and mixed using inert mill balls. After the termination of the mixing process, the mill balls however have to be removed from the pastelike active material composition, which is difficult. Adhesion to the surfaces of the mill also causes material losses and production standstills.
Due to their specific method steps, the described production methods are inadequately suited for the preparation of material compositions having a high degree of homogeneity without placing excessive mechanical loads on the components. Frequently, the formation of agglomerates is observed, in particular the formation of binding agent agglomerates.
U.S. Pat. No. 7,087,348 B2 describes a method in which first the surface of the active material is filled at least 50%, preferably at least 75%, with a composition that includes an electrically conductive material. This enables a good electrical connecting of the active material to the current collector. Disadvantageous here is the interaction, made more difficult by the large-surface occupation of the surface of the active material particles, of the active material with the charge bearers (in particular the lithium cations) from the electrolyte composition during the operation of the electrode.